Unit of weight used by masons (5)
I believe the answer is:
stone
This might be a double definition.
'unit' is the first definition.
(stone is a kind of unit)
'weight used by masons' is the second definition.
'stone' can be an answer for 'weight' (stone is a unit of weight). I am not certain of the 'used by masons' bit.
'of' is the link.
Can you help me to learn more?
(Other definitions for stone that I've seen before include "14 lb" , "Rag" , "Rock; fruit seed" , "Rock; gem" , "Rock; hard seed" .)